Digital to analog converters (DAC) are used to convert a digital representation of a signal into an analog representation of the same signal. DACs are used in a wide variety of applications, ranging from medical and entertainment to communications (both voice and data). Digital to analog converters are electrical circuit devices that convert a digital signal that is a series of multi-bit samples, or numbers, in the digital domain to a continuous signal, such as a voltage or current, in the analog domain. A variety of DAC converter types exist, including a thermometer DAC, R-2R ladder network DAC, segmented DAC, oversampling/interpolating DAC, and pulse-width modulated DAC, for example. Another type is known as a sigma delta or delta sigma (ΔΣ) D/A converter. It consists of an “interpolation filter” that is a digital circuit which accepts data at a low rate, inserts zeros at a high rate, and then applies a digital filter algorithm and outputs data at a high rate, a ΣΔ modulator that effectively acts as a low pass filter to the signal but as a high pass filter to the quantization noise, and converts the resulting data to a high speed bit stream, and a 1-bit DAC whose output switches between equal positive and negative reference voltages. The output is filtered in an external analog low pass filter (LPF). It is also possible to use more than one bit in the ΣΔ DAC.
The oversampling in a sigma delta DAC is commonly performed at a multiple of the Nyquist rate (FN) for a given input signal frequency and typically the sampling frequency FS is 10 to 1000 times FN. In this manner, quantization noise power is spread over a bandwidth equal to the sampling frequency, thereby reducing the noise density in the band of interest. Sigma-delta DACs are commonly used in applications where high resolution with low to moderate conversion rates is required. An advantage of sigma-delta DACs is that the sigma-delta DACs normally make use of single or low multi-bit (typically two, three, four or six bit) quantizer, making the precision requirements of the sigma-delta DAC much lower than other types of DACs that may use quantizers with a large number of bits. However, sigma-delta DACs having a larger number of elements, such as 32 or 64 elements, are now becoming more common. Operating at a frequency greater than the required frequency is commonly referred to as oversampling and a DAC that is operating at a frequency that is K times greater than the required frequency is referred to as a K-times oversampling DAC.
Mobile audio devices are a ubiquitous fixture of modern society. Cellular telephones, personal music players, portable gaming systems, etc. are constant companions for many people. Music players and gaming systems may make use of ΣΔ DACs to produce the audio signal(s) that are then reproduced by a speaker. Cell phones continue to increase in computer processing capability and sophistication. The basic radio transceiver within the cell phone may make use of a sigma-delta DAC for signal modulation and transmission. The increased memory capacity and computing resources on a cell phone support the installation of various applications, often referred to as “apps” that allow a diverse range of functions to be performed by the cell phone when not being used for conversation. Digital to analog conversion of audio signals to drive speakers/headsets is required by several apps that run on a mobile device and may be performed by a sigma-delta DAC.